This invention relates to communication receivers and particularly to techniques for measuring a signal-to-interference ratio. The invention is useful in wireless telecommunication systems, especially mobile communication systems.
In wireless communication systems, co-channel interference is a principal source of performance degradation as well as of system capacity limitations. When a wireless system is used in a cellular environment, a terminal unit and a base station are needed to monitor the system performance and to use that system performance as the criterion for hand-off from one cell to another cell. A wireless communication channel may be limited by either noise or interference. The parameters which are usually used for the hand-off criteria are the receive signal strength, bit error rate, and signal-to-interference ratio (S/I). What is needed is a mechanism for accurately measuring those parameters.
According to the invention, in a receiver and in particular in a nonlinear receiver, a method and apparatus are provided for performing a signal-to-interference ratio measurement wherein detected information is used to estimate the interference power and a ratio calculation is made on signal power and interference power to yield a desired signal-to-interference ratio. Both the received signal and detected signal are used in the measurement. The received signal is used to calculate the accumulated power of the signal, noise, and interference. The signal power measurement of the received signal is determined by squaring the signal strength of the received signal. Simultaneously, the detected signal, that is the signal after the detection process has been undertaken, i.e., a reconstructed version of the signal from the detected information, is correlated with the received signal in order to provide a signal free of the interference and noise elements in the source signal. The power of the desired signal is calculated by squaring the output of the correlator. By taking the difference between the received signal power and the signal power of the desired signal, the power of the interference plus noise is calculated. When the channel is interference-limited, the power of the noise is negligible. Therefore, by dividing received signal power by the interference plus noise power, the signal to interference (S/I) ratio is calculated.
In a particular embodiment, a correlator receives as its inputs signal samples from a main receiver and so-called detected information produced by both the main receiver and a diversity receiver, where detected information is information expected to be received at the two receivers and which has been detected and mapped according to a preselected modulation format similar to the transmitted signal. This processed signal is then used to measure the power of the desired signal as it should have been received.
While, in one specific embodiment, the detector which produces one of the signals to the correlator uses as its inputs the received signals from both main and diversity receiver paths, in another specific embodiment the detector processes only the signal from a single receiver.
The invention is useful with hardlimited as well as linear received signals, and it can be used in receivers that receive continuous and burst signals. The accuracy of input measurement may be improved by increasing the measurement time.
The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.